1. Field of the Invention
The present invention relates to an externally attached handset of an end terminal used for mobile communication systems, and more particularly to an externally attached handset of an end terminal used for a portable radiotelephone or an automobile telephone connected with a facsimile machine.
2. Discussion of the Background
In recent years, mobile communication systems, for example, portable radiotelephones and automobile telephones, have become widely used. With the spread of such systems, demand for facsimile communication via mobile communication systems has been increasing. Accordingly, a NCU (network control unit) used for connecting a data communication terminal, such as a facsimile device that communicates non-audio data, to a portable radiotelephone or an automobile telephone has become commercially available.
In such NCUs, one for automobile telephones adopts a system wherein a NCU is connected between a radio device (a main body of an automobile telephone) and a handset. By this connection, regular telephone conversations can be done by using a handset, and transmission and reception of a facsimile can be performed by turning on the facsimile machine and turning on the communication start button during the telephone conversation, if necessary.
On the other hand, a NCU used for portable radiotelephones is different from that for the automobile telephones. The NCU cannot be connected between a radio device and a handset since the radio device and the handset of the portable radiotelephone are incorporated in one body. For this reason, a facsimile device is connected to the handset of the portable radiotelephone via a speaker/microphone terminal equipped for connecting a headphone so as to perform transmission and reception of facsimile data.
FIG. 1 shows an example of a conventional portable radiotelephone with a headphone connected to a speaker/microphone terminal. Regularly, in a state where a headphone 14 is not connected, conversation is made with a speaker 13 and a microphone 12 provided on the portable radiotelephone 11. The portable radiotelephone 11 has a jack 15 (speaker/microphone terminal) for connecting a plug of the headphone 14. When the plug of the headphones 14 is inserted into the jack 15, the speaker 13 and the microphone 12 on the portable radiotelephone 11 switch off and telephone conversation can be done only with a speaker 16 and a microphone 17 of the headphones 14.
When connecting a data communication end terminal, such as a facsimile machine, to the portable radiotelephone 11, as shown in FIG. 2, commercially available NCU 21 for portable radiotelephones are required. The NCU 21 may have a jack for connecting the headphones 14, as shown in the figure, or a monitor speaker, not shown in the figure. Major functions of the NCU 21 are two-wire/four-wire conversion, transmission level adjustment, reception level adjustment, impedance matching, TEL/FAX switching and detection of closed loop, explained in the following.
As is well known, two-wire/four-wire conversion is required, as the signal line at the jack 15 of the portable radiotelephone 11 is a four-wire one while a regular facsimile machine adopts a two-wire line for input.
The closed loop detection is for switching the signal line to either TEL or FAX by detecting a DC current present when a closed loop for facsimile line is formed. The closed loop is formed when a handset of the facsimile machine is taken off hook or when a TEL/FAX switch on the facsimile device is switched to FAX. FIG. 3 shows a block diagram of a part of the NCU 21, which part performs a switching of the TEL/FAX functions. Usually, a relay 23 is switched to the TEL side and telephone conversation is performed via the headphones 14. A controller 24 continues to detect a formation of the closed loop during communication via conversation.
As shown in FIG. 4, a relay R on the facsimile machine is usually switched to the TEL side. The relay R is switched to FAX when the handset of the facsimile machine is taken off hook or when the TEL/FAX switch of the facsimile machine is switched to FAX. If the closed loop is detected, it is judged that facsimile communication is started and the relay 23 is switched to the FAX side. However, regular facsimile machines used for mobile communication systems do not have a handset because portable radiotelephones and automobile telephones include handsets.
FIG. 5 is a flow chart of operation of a switching function performed by the NCU 21. In step P1, the controller 24 checks whether or not a closed loop is formed, by detecting whether or not a DC current is present at a jack to which the facsimile machine is connected. If it is judged that the closed loop is formed, the procedure proceeds to step P2 where the relay 23 of the NCU 21 is switched to the FAX side. In step P3, it is judged whether or not the closed loop is still formed. If the closed loop is not formed, the relay 23 is switched to the TEL side in step P4, and the procedure ends.
However, the above mentioned conventional system has a disadvantage in that an operation for using a facsimile machine is complicated. That is, the NCU 21 is allowed to be connected only when facsimile communication is to be performed, and after the connection a start switch of the facsimile machine has to be operated. This is because when the NCU 21 is connected via the jack 15, the speaker 13 and the microphone 12 of the portable radiotelephone 11 are off the line, which leads to a situation such that telephone conversation cannot be performed.
For example, as shown in FIG. 2, in case using the NCU 21, which can be used with the headphones 14, when starting facsimile communication, firstly a plug of the connection line of the NCU 21 is inserted into the jack 15 of the portable radiotelephone 11. After that, the user must put the headphones 14 on his head and conduct a conversation. When completion of preparation for facsimile communication is confirmed with the opposite side, the user must turn the start switch on. This procedure is very inconvenient for the user.
Additionally, if facsimile communication is started by mistake, time elapses until the facsimile communication ends and thus is wasted. This is because while the system is in a facsimile communication mode, the relay 23 is switched to the FAX side, which does not permit monitoring the communication by the headphone 14.
In case a NCU having a monitor speaker is used, a voice from the remote party can be heard but a voice from one's own side cannot be sent to the remote party. Accordingly, it is required that a plug be inserted each time when performing facsimile communication, and thus a disadvantage still exists.